What is this line coming from the light source and how do I adjust it?
That line indicates the direction of a sun lamp.
If you rotate it it will determine the direction for the light and shadows.
Note that for sun lamps the placement in the 3d environment is irrelevant, they just have direction.
To control the size of the line (and it is just an informative line line for you to see the direction) just scale it (press S and drag the mouse)